Fair use limits the exclusive rights of the copyright holder, and is especially helpful in academic settings. Specifically, the Copyright Act of 1976 refers to uses such as “criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research.”
However, “fair use” is described in terms of guidelines and not with specific rules and is therefore open for interpretation. Section 107 of the Copyright Act of 1976 provides the following four principles.
All four of these factors must be considered and weighed for each situation, although courts have historically put the most weight on the first and fourth factors, especially the fourth.
Careful consideration of the four factors is absolutely necessary, and you will want to keep a record of your decision making in case you are challenged. Columbia University Libraries' Fair Use Checklist can be helpful for this record keeping.
(exposition based on U.S. Copyright Office More Information on Fair Use, 2017 and Crews, K.D. Copyright Law for Librarians and Educators, Chicago: American Library Association, 2006):
Items in the public domain are not eligible for copyright protection and may be reproduced without restriction. These include:
Implied Licenses for Internet Materials
Contrary to what some believe, copyright does still apply to materials on the Internet. Just because an author posts something on a Web site does not mean that the material is automatically in the public domain.
That said, because of the nature of the Internet, there is something of an implied license for these materials. When an author makes something available on the open Internet (as opposed to proprietary Internet, those sections specifically licensed for sale and protected behind passwords such as most library subscriptions), he or she should expect that people will read, download, print-out, forward, and use that material as the basis for other works to a certain degree. Therefore there is an implied license to conduct these sorts of activities.
As with many other areas of copyright, the boundaries for these implied licenses are not entirely clear, and the word “reasonable” tends to come up as the measuring stick for gauging infringement.
It is always good to keep in mind that in copyright cases the courts seem to look most closely at whether the use of copyrighted materials has diminished the market for those materials. Is the copyright holder losing money due to the actions taken by the user of the materials? This is especially important when talking about peer-to-peer file sharing.